The present invention relates to an iron-based alloy sheet which is useful as a material of magnetic yokes in a compact-size flat-type hard-disk voice-coil motor (VCM) as well as to a magnetic yoke manufactured from such an iron-based alloy sheet capable of exhibiting a high magnetic flux density as a component of the magnetic circuit.
Voice-coil motors are widely employed as an actuator for driving a hard-disk head in which, as is known, the magnetic circuit of the voice-coil motor is constituted of permanent magnets and magnetic yokes connecting the permanent magnets. Along with the trend in recent years that computers are required to be more and more compact in size and lighter and lighter in weight in consideration of good port-ability and convenience in other respects, the magnetic recording instruments are designed also to be compact and flat. This trend toward compactness and flatness of instruments is now extending influences thereof to the permanent magnets and yokes constituting the magnetic circuit of voice-coil motors.
A conventional way to accomplish further compactness and flatness of a magnetic circuit is to use a high-performance permanent magnet capable of giving an increased magnetic flux density by which the decrease in the magnetic flux density in the air gap space due to volume decrease can be compensated.
While high-performance permanent magnets are on the way of further improvements year by year to generate a further increased magnetic flux density, the magnetic yokes in voice-coil motors are manufactured conventionally from a magnetically soft rolled steel sheet of the grade such as SPCC, SPCD, SPCE and others so that, insofar as the magnetic yokes manufactured from these conventional steel sheets are used, improvements can hardly be expected in the saturated magnetization of the magnetic yokes in compliance with upgrading of the magnetic flux density of the permanent magnets. Since the thickness of the magnetic yokes is also under limitation by the compactness or flatness of the instrument as a whole, a large magnetic flux of a high-performance permanent magnet cannot be fully utilized in the magnetic circuit with local saturation of the yokes or partial leakage of the magnetic flux out of the magnetic circuit.
The disadvantage caused by the magnetic flux leakage mentioned above is not limited to the undesirable decrease of the magnetic flux density in the air gap within the magnetic circuit but also includes adverse influences and disturbances on the magnetic recording media and other peripheral instruments for control of the system. Accordingly, current product standards specify that the amount of magnetic flux leakage must be kept low not to exceed the specified upper limit.
The magnetic material of magnetic yokes in magnetic recording instruments and other instruments most widely used in recent years includes cold-rolled steel sheets of the grade such as SPCC, SPCD, SPCE and others by virtue of their excellent workability in punching, templating, drilling, bending and embossing as well as of their inexpensiveness. Since the saturation magnetization of these steel sheet materials is not very large, however, it is difficult, in a VCM magnetic circuit in compliance with the above mentioned trend toward compactness and flatness, to completely avoid local saturation of magnetization and to fully conduct the magnetic flux coming from high-performance permanent magnets generating a high magnetic flux density through the magnetic circuit.
In view of the above described problems and disadvantages in the prior art magnetic yokes in hard-disk voice-coil motors, the present invention has an object to provide an iron-based alloy sheet suitable as a material for the manufacture of magnetic yokes in a voice-coil motor by which any high magnetic flux density provided by a high-performance permanent magnet can be fully utilized in the magnetic circuit without leakage of the magnetic flux out of the magnetic circuit.
Thus, the iron-based alloy sheet provided by the present invention is a sheet material having a thickness in the range from 0.1 mm to 5.0 mm and is made from an iron-based alloy which comprises inherent side elements including:
from 0.0001 to 0.02% by weight of carbon;
from 0.0001 to 0.05% by weight of silicon;
from 0.001 to 0.2% by weight of manganese;
from 0.0001 to 0.05% by weight of phosphorus;
from 0.0001 to 0.05% by weight of sulfur;
from 0.0001 to 0.1% by weight of aluminum;
from 0.001 to 0.1% by weight of oxygen; and
from 0.0001 to 0.03% by weight of nitrogen,
the balance to 100% by weight being iron and unavoidable impurity elements, and which is characterized by the magnetic parameters including:
a saturated magnetic flux density in the range from 2.07 to 2.2 Tesla;
a maximum magnetic permeability in the range from 1000 to 20000; and
a coercive force in the range from 10 to 400 A/m.
Further improvements in the magnetic properties can be accomplished when the above defined alloy composition is additionally admixed with 0.1 to 10% by weight of cobalt to give:
a saturated magnetic flux density in the range from 2.07 to 2.3 Tesla;
a maximum magnetic permeability in the range from 1000 to 20000; and
a coercive force in the range from 10 to 400 A/m.
When further upgrading of the performance of the iron-based alloy sheet for magnetic yokes is desired, the iron-based alloy can further comprise from 0.1 to 10% by weight of cobalt as an adjuvant element or from 0.01 to 5% by weight as a total of an adjuvant element or elements selected from the group consisting of titanium, zirconium, niobium, molybdenum, chromium, vanadium, nickel, tungsten, tantalum and boron in combination with up to 10% by weight of cobalt or without cobalt so that the magnetic parameters characterizing the iron-based alloy include:
a saturated magnetic flux density in the range from 2.07 to 2.3 Tesla;
a maximum magnetic permeability in the range from 1200 to 22000; and
a coercive force in the range from 20 to 380 A/m.